When asked how much sushi you can eat in a week, your answer might be “um, as much as you’ll let me” — and trust us; we’re with you. From sushi tacos to sushi cakes, there’s an abundance of raw fish recipes on our radar. But there is a very real limit to how much of the stuff you can safely ingest. After six servings, it’s time to turn to a fish-free option, according to Dr. Roshini Raj on CNN.

Fish is super nutritious, no doubt about it. It’s high in protein, omega-3s, B vitamins, and other nutrients, making it a very heart-healthy protein source. In fact, the American Heart Association recommends consuming fish at least twice a week. Where you can run into problems is with fish’s mercury content. These days, many fish are raised in or harvested from polluted waterways and are therefore high in mercury, a heavy metal that’s extremely toxic when consumed in large doses.

Unfortunately, many popular sushi fish are notoriously high in mercury, particularly tuna, mackerel, yellowtail, swordfish, and sea bass. These large fish have consumed lots of smaller fish over the course of their lifetimes, ingesting more mercury than the little guys. The National Resources Defense Council has a handy chart of the fish with the least and most amounts of mercury (e.g., opt for oysters and salmon over tuna).

Sushi also raises concerns about dioxins, which — though well-regulated in the US — are less stringently controlled in Asian waters. Dioxins are carcinogens, and, like mercury, are often present in fish due to the toxic nature of the waters in which the fish are raised.

We know this all sounds scary, but sushi can be a fantastic component of an extremely healthy diet. You just don’t want to eat too much of it (sorry, you shouldn’t eat it every day). Trace amounts of these substances aren’t generally problematic — like everything, moderation is key. Be mindful of the types of fish you consume, try to eat sustainable seafood when possible, and consult a doctor if you’re ever unsure.

How often do you eat sushi? Tweet us at @BritandCo and tell us!